In the past few years, there has been a considerable effort to develop new sources of electrical power. Included has been the development of systems primarily designed for providing power to a single enterprise, for example, a household, with any excess generated power being fed back to a power line of a public utility providing a primary source of power for that household. Frequently, the auxiliary or local power generating unit is in the form of a windmill, and there are times when little or insufficient power is available from it alone. Thus, as a matter of convenience, in order to preserve a continuous interconnection of power to on-site electrical devices to be powered, the windmill and public utility power lines are connected together.
Windmill generators have typically been of the direct current type, and thus in order to achieve compatibility with public power lines, which are of alternating current powder, the output of such a generator must be converted to alternating current power. This is accomplished by switching means operating synchronously with the frequency, typically 60 cycles, of the power line. In addition to effecting frequency compatibility, there must be both voltage amplitude and phase compatibility between the generated output and the power line voltage. All in all, such a coupling system is necessarily complex and costly.
As an alternate to the direct current generator, induction motor/generator units are sometimes used with windmill generating systems. While the induction motor/generator has not seen great use as a generator in the past, it is perhaps the most widely used type of motor, and thus is widely available and at a reasonable cost.
The power input to an induction motor is given by the product of the applied voltage, the current, and the cosine of the phase angle between the voltage and current (E I Cosine a). In a heavily loaded motor, the current will tend to be in phase with the voltage. When unloaded, the current will typically lag the voltage 70.degree. to 80.degree.. If an external force tends to drive the shaft higher than synchronous speed, the phase lag will continue to increase. When the force is sufficient to cause the phase lag to be 90.degree., the power input to the motor is zero since cosine 90.degree.=0. At this point, the mechanical energy applied to the shaft is exactly equal to the magnetizing losses, and there is no net energy being returned to the A.C. buss. As the driving force continues to increase, the phase angle becomes greater than 90.degree.. The cosine of angles greater than 90.degree. is negative, indicating negative power flow. The motor is now generating and returning energy to the A.C. buss. Further increase in driving force causes the phase lag to approach 180.degree. as the full generatiing capacity of the machine is reached.
Significantly, the induction generator requires no synchronization or voltage regulation circuitry to couple its output to a power line. It inherently functions as a generator when it is driven above its synchronization speed, a speed equal to the frequency of the power line divided by the number of pairs of poles that it contains, typically the speed being 1,800 rpm in the case of a 4-pole device. It, like a direct current generator, is typically connected to a power line when its speed is sufficient for the production of power which, in the case of the induction motor/generator, is at sync speed. Beyond this speed, and in the range of approximately five percent of the sync speed, this type device provides increasing power output to a power line, this increase occurring as the phase lag of current with respect to voltage increases above 90.degree., an angle which persists at the sync speed.
Despite the obvious advantages of the induction motor/generator over a D.C. generator as described, the former has one significant disadvantage. It must draw field excitation power from the power line that is connected to it. This excitation current is drawn during a portion of each half cycle of the A.C. line voltage when current and voltage are of the same polarity, which, in the case of a lightly driven generator, is only slightly less than one-half of each half cycle. Thus, in such case, it can only function as a generator during the remaining slightly more than one-half of each half cycle, and thus its net output as a generator is essentially slight. At higher speeds, the ratio of power drawn to power delivered improves.
It is the object of this invention to effectively reduce the portion of each half cycle where current is drawn by the generator, and thus substantially improve its efficiency, particularly at low velocity drive levels which, in the case of windmill operation, may persist for a substantial portion of the time of operation.